1. Field of the Invention
The present invention relates to RF (Radio Frequency) power dividers and, more particularly, to a Wilkinson power divider used in a UHF (Ultra High Frequency) band high-power amplifier of a radio transmitter.
2. Description of the Related Art
As various radio paging services are provided, repair and management have emerged as significant issues to service providers. This is especially so since ever increasing radio pager subscribers have resulted in a proportionate increase of the number of radio paging transmitters utilized to transmit a signal via an air path. However, service providers cannot limitlessly increase radio paging equipment, let alone personnel and subsidiary installations.
Thus, since signals in various forms are rapidly transmitted at one time, and radio paging transmitters broadcast signals ever farther, it is desirable that adequate services be provided with present personnel and established radio paging equipment and subsidiary installations, regardless of the increasing number of subscribers. Accordingly, the need for a high-power amplifier that is able to transmit a signal over a wider area is ever pressing.
Meanwhile, most high-power amplifiers obtain a high power output by combining a plurality of amplifying devices in parallel to amplify a radio signal. However, such combinations are not without output limitations. A device for dividing or combining radio signals is called an RF power divider or combiner. RF power dividers are categorized into T-junction power dividers, Wilkinson power dividers, and quadrature hybrid power dividers. They are selectively used in accordance with their characteristics.
These RF power dividers are used in radio transmitters such as the aforementioned radio paging transmitters. Among them, the Wilkinson power divider is widely used in UHF-band radio transmitters and is basically comprised, in terms of transmission lines, as shown in FIG. 1. FIG. 1 illustrates an equal split ratio Wilkinson power divider for dividing an input signal into two equal output signals. Here, the output signals are in phase with, and 3 dB smaller than, the input signal.
The Wilkinson power divider of FIG. 1 is generally implemented with microstrip lines or coaxial lines having corresponding impedances on a substrate, as shown in FIG. 2. The RF power divider of FIG. 2 has a single input port IP1, and first and second output ports OP1 and OP2. A microstrip line 10 is coupled to input port IP1, and two microstrip lines 18 and 12 are serially coupled between an output side of microstrip line 10 and first output port OP1. Two microstrip lines 20 and 14 are serially coupled between the output side of microstrip line 10 and second output port OP2. Assuming that a system impedance is Z.sub.0, the respective impedances of the microstrip lines 10, 12, and 14 are Z.sub.0, as shown in FIG. 1. The impedances of microstrip lines 18 and 20, which act as quarter wavelength (.lambda./4) lines, are .sqroot.2Z.sub.0, respectively. In addition, a parallel resistor 16 is coupled between the contact points of microstrip lines 18 and 12, and of microstrip lines 20 and 14. The resistance of resistor 16 is 2Z.sub.0, as shown in FIG. 1.
When the .lambda./4 lines are implemented with microstrip lines or coaxial lines as described above, the length of the respective divided lines corresponds to a .lambda./4 of the frequency band for which power is divided. That is, the length of microstrip lines 18 and 20 is a .lambda./4 of the frequency band and thus, becomes very large if the frequency band is a UHF band. For example, a .lambda./4 transmission line for 325 MHz is about 23 cm long.
Therefore, when .lambda./4 lines are implemented with microstrip lines or coaxial lines, the RF power divider occupies a large portion of a given amplifier area. As a result, spatial constraints are imposed on the remaining amplifier circuits. Further, the .lambda./4 transmission line, the length of which is set in accordance with the frequency band used, is useless against external condition changes such as frequency variation. Thus, the RF power divider should be reconstituted under the varied condition in order to obtain the intended electrical properties.
Another issue with respect to RF power dividers is the measuring of a divider's output signal(s). In order to measure an output signal, measurement terminals of a measuring instrument should be directly connected to the input and output ports. Conventionally, the measurement terminals have been directly soldered to the input and output ports.